NAIROBI, Kenya - A plan to merge Somalia's two main insurgent groups was announced Sunday, bringing together a bloc that has vowed allegiance to Osama bin Laden and one that has opposed an al-Qaeda affiliation.

Both groups are fighting the weak, Western-backed government, whose writ runs mainly in the capital Mogadishu.

The Islamic Party's head of operations, Sheikh Mohamed Osman Arus, said it will join forces with the al-Qaeda-linked al-Shabab militia and fight under al-Shabab's name.

Fighters from the Islamic Party have been defeated several times by al-Shabab, including a battle to control the port city of Kismayo.

But Arus denied the merger was a face-saving tactic for his weakened group, saying the planned union would be a way to influence al-Shabab hard-liners from within.

He said the merger would soon be formally announced, but did not give a date. Al-Shabab spokesmen were not available for comment.

The Islamic Party has previously condemned al-Shabab's use of tactics, including suicide bombings and summary executions. Its founder, Sheikh Hassan Dahir Aweys, has also criticized al-Shabab's pledging of allegiance to bin Laden.